Evolutionary Genetics Field Trip Group members: Mallory Bedford * Jack Brennan Kelly Davies Alex Laybourn James Cullum
Introduction
The genus Cepaea consists of four species of snail, including Cepaea nemoralis. This organism is the focus of this investigation because it exhibits polymorphism, which is variation in the genotype that arises by mutation and is directly reflected in the phenotype. Each morph of C. nemoralis is easily distinguishable from the others; individuals of this species can have a background shell colour of pink, brown or yellow and can have any number of bands between none and five. This is one of the reasons that this organism was selected for this investigation rather than other organisms, such as humans. Even after death the morphological traits are maintained, and therefore the individual can still be analysed in terms of their alleles. The populations tend to remain isolated due to their low mobility (5-10 m/year), making the effects of genetic processes on different populations easily comparable. Previous investigations into this species have provided a great depth of background knowledge and contribute valuable information for use in future studies as, here is a example from Jones: “The relative importance of the various mechanisms will vary from population to population, but almost nowhere it will be possible to explain the observed pattern of genetic variation by invoking only one of them” (Jones et al.). These organisms, although anatomically simple and sluggish, show exactly the same patterns in terms of distribution and genetic processes as more complicated organisms. C. nemoralis are not difficult to catch which makes them they are an ideal candidate for this study.
The aim of this investigation was to analyse the degree of influence of genetic drift and genetic selection on the allele frequencies of the micro-populations that were present on Pulpit Hill. Hence, determining if one of these processes had a more significant impact upon the populations than the other. There are many factors which affect these populations, these include the availability of food, shelter, human disturbance & disruption, soil acidity, climatic conditions, inter & intra specific competition and the abundance of predators. A relationship between predation and allele frequency has been shown as “an allele will be favoured when it is rare because predators are unlikely to encounter it when they first sample the population” (Jones et al.)All of these factors can relate to the availability and quality of food, altitude, egg laying and rate of reproduction. The impact of these was gauged through statistical analyses such as the Chi squared test. Previous studies (e.g. Jones et al.) have concluded that selection by predation and environmental pressures is the driving force behind fluctuations in the allele frequencies in C. nemoralis. Studies did concede that there was certainly incidence of genetic drift within these populations. Our aim was to perform similar analyses and see whether our results reflected those performed previously. We achieved this by designing and conducting our own experimental methods and analysing the results.
Our methods involved first choosing different sites of the same size at three different altitudes (high, medium and low) and in two different vegetative habitats (grass and shrub). This allowed a wide range of results, and each site was sufficiently far from the others (>10 metres which was the maximum estimated yearly movement of these organisms) to contain an isolated population, which would allow for accurate comparisons of genetic processes and allelic distributions within these populations. We aimed to sample from isolated populations as a “random loss of alleles must occur in any population which is sufficiently small and isolated” (Jones et al.). Our sampling method was chosen to acquire a representative sample of the population contained therein as a whole.
The hypothesis that we were investigating was that ‘genetic selection has a larger influence than genetic drift on the allele frequencies of C. nemoralis in the populations that we analysed’. Another prediction is that there will be a larger number of C. nemoralis in the shrub rather than open grassland. If the chi squared test shows a statistical significance this would imply that there is significant genetic difference between the populations that we studied, suggesting that either drift or selection was taking place.
Reference:
J. S. Jones, B. H. Leith, P. Rawlings (1977) Polymorphism in Cepaea: A Problem with Too Many Solutions? Annual Review of Ecology and Systematics, 8:109-143
Discussion
The samples were taken at seven different sites on Pulpit Hill. They were chosen to compare the impact of gradient and vegetation on the populations of C. nemoralis in this locality. Pulpit hill has a sufficient gradient difference between high and low that this is directly reflected in the pH of the soil. The pH affects the ability of the snails to reproduce, because they put their eggs in the ground, and therefore if the soil is too acidic or alkaline, it could have a detrimental effect on the populations living there. The geology of the area, which is primarily chalk, is also important. This is because the snails use the calcium carbonate to form their shells, and runoff of rain affects the pH of the soil.
The sample areas were divided between two separate habitats – shrub and grass. This was to determine if the clear differences between these two types of vegetation had an equally obvious effect on the genetic distribution of C. nemoralis. However, from statistical analyses of the data collected, there as no significant difference in the allele distribution between shrub and grass. This was determined with a chi squared test (χ2 = 0.039, P = N/S). One of the major factors which contributed to this result was sample size. This is likely to have had an effect on the significance of this relationship because our samples were not large enough to be representative of the whole population. Another possible error that could have led to this result was disturbance at the sites from human interference, especially because it was likely that another group might have previously sampled from there. This is because the sites from which we sampled were ideal, and therefore were likely to be tampered with. The differences between these habitats made it likely that sampling error played a part in this result, because snails were much easier to find on the short grass, and harder to see amongst the shrub. These factors may be reasons for the lack of difference shown, but it is possible that there simply are no genetic differences between the populations found in grass and shrub on this hill.
Further chi squared tests showed that there were no significant differences in allele frequency between the populations found in the high ground and in the low ground. This again may be due to one of many factors, but equally may simply show that there are no genetic differences between these populations.
At the high ground, we chose to sample the shrub areas with two different sampling methods, to see if this had any effect on the number and type of snails that we were able to find. The two sampling methods (See methods section) involved different placements of the line transects relative to one another. By statistically comparing the populations sampled at these, it was determined that there was a significant difference in the allele frequencies (χ2 = 10.06, P = 0.01). It is also significant when Yate’s correction is applied, because there is only one degree of freedom (χ2 = 8.27, P = 0.01). This could be due to the fact that the sampling methods were different, and the first covered less area than the second. In this way, human error and the skewed method may have contributed to the apparent difference in allele distribution. Our group was also the first to sample at the first site, eliminating interference from other groups, but it was possible that other groups had previously sampled at the second site.
However, the difference may be explained by the occurrence of genetic drift or selection. The sites were sufficiently far apart (distance between sites > average snail movement) that it can be concluded that the populations are isolated from one another. There were identifiable differences in the composition of the vegetation at each site. At the first site there were generally shorter plants with more soil disturbance, exposing the chalk, at the second site the plants were taller and there was a greater coverage of vegetation. These differences in conditions give ways in which selection might act upon the populations. For example the greater coverage of plants at the second site would give greater protection from predators and the accessibility of chalk at the first site would better enable shell development. It is almost certain that genetic drift and selection is action on both populations but without further investigation we cannot determine whether drift or selection has a greater effect on allele distribution and frequency.
Group members:
Mallory Bedford *
Jack Brennan
Kelly Davies
Alex Laybourn
James Cullum
Introduction
The genus Cepaea consists of four species of snail, including Cepaea nemoralis. This organism is the focus of this investigation because it exhibits polymorphism, which is variation in the genotype that arises by mutation and is directly reflected in the phenotype. Each morph of C. nemoralis is easily distinguishable from the others; individuals of this species can have a background shell colour of pink, brown or yellow and can have any number of bands between none and five. This is one of the reasons that this organism was selected for this investigation rather than other organisms, such as humans. Even after death the morphological traits are maintained, and therefore the individual can still be analysed in terms of their alleles. The populations tend to remain isolated due to their low mobility (5-10 m/year), making the effects of genetic processes on different populations easily comparable. Previous investigations into this species have provided a great depth of background knowledge and contribute valuable information for use in future studies as, here is a example from Jones: “The relative importance of the various mechanisms will vary from population to population, but almost nowhere it will be possible to explain the observed pattern of genetic variation by invoking only one of them” (Jones et al.). These organisms, although anatomically simple and sluggish, show exactly the same patterns in terms of distribution and genetic processes as more complicated organisms. C. nemoralis are not difficult to catch which makes them they are an ideal candidate for this study.
The aim of this investigation was to analyse the degree of influence of genetic drift and genetic selection on the allele frequencies of the micro-populations that were present on Pulpit Hill. Hence, determining if one of these processes had a more significant impact upon the populations than the other. There are many factors which affect these populations, these include the availability of food, shelter, human disturbance & disruption, soil acidity, climatic conditions, inter & intra specific competition and the abundance of predators. A relationship between predation and allele frequency has been shown as “an allele will be favoured when it is rare because predators are unlikely to encounter it when they first sample the population” (Jones et al.)All of these factors can relate to the availability and quality of food, altitude, egg laying and rate of reproduction. The impact of these was gauged through statistical analyses such as the Chi squared test. Previous studies (e.g. Jones et al.) have concluded that selection by predation and environmental pressures is the driving force behind fluctuations in the allele frequencies in C. nemoralis. Studies did concede that there was certainly incidence of genetic drift within these populations. Our aim was to perform similar analyses and see whether our results reflected those performed previously. We achieved this by designing and conducting our own experimental methods and analysing the results.
Our methods involved first choosing different sites of the same size at three different altitudes (high, medium and low) and in two different vegetative habitats (grass and shrub). This allowed a wide range of results, and each site was sufficiently far from the others (>10 metres which was the maximum estimated yearly movement of these organisms) to contain an isolated population, which would allow for accurate comparisons of genetic processes and allelic distributions within these populations. We aimed to sample from isolated populations as a “random loss of alleles must occur in any population which is sufficiently small and isolated” (Jones et al.). Our sampling method was chosen to acquire a representative sample of the population contained therein as a whole.
The hypothesis that we were investigating was that ‘genetic selection has a larger influence than genetic drift on the allele frequencies of C. nemoralis in the populations that we analysed’. Another prediction is that there will be a larger number of C. nemoralis in the shrub rather than open grassland. If the chi squared test shows a statistical significance this would imply that there is significant genetic difference between the populations that we studied, suggesting that either drift or selection was taking place.
Reference:
J. S. Jones, B. H. Leith, P. Rawlings (1977) Polymorphism in Cepaea: A Problem with Too Many Solutions? Annual Review of Ecology and Systematics, 8:109-143
Discussion
The samples were taken at seven different sites on Pulpit Hill. They were chosen to compare the impact of gradient and vegetation on the populations of C. nemoralis in this locality. Pulpit hill has a sufficient gradient difference between high and low that this is directly reflected in the pH of the soil. The pH affects the ability of the snails to reproduce, because they put their eggs in the ground, and therefore if the soil is too acidic or alkaline, it could have a detrimental effect on the populations living there. The geology of the area, which is primarily chalk, is also important. This is because the snails use the calcium carbonate to form their shells, and runoff of rain affects the pH of the soil.
The sample areas were divided between two separate habitats – shrub and grass. This was to determine if the clear differences between these two types of vegetation had an equally obvious effect on the genetic distribution of C. nemoralis. However, from statistical analyses of the data collected, there as no significant difference in the allele distribution between shrub and grass. This was determined with a chi squared test (χ2 = 0.039, P = N/S). One of the major factors which contributed to this result was sample size. This is likely to have had an effect on the significance of this relationship because our samples were not large enough to be representative of the whole population. Another possible error that could have led to this result was disturbance at the sites from human interference, especially because it was likely that another group might have previously sampled from there. This is because the sites from which we sampled were ideal, and therefore were likely to be tampered with. The differences between these habitats made it likely that sampling error played a part in this result, because snails were much easier to find on the short grass, and harder to see amongst the shrub. These factors may be reasons for the lack of difference shown, but it is possible that there simply are no genetic differences between the populations found in grass and shrub on this hill.
Further chi squared tests showed that there were no significant differences in allele frequency between the populations found in the high ground and in the low ground. This again may be due to one of many factors, but equally may simply show that there are no genetic differences between these populations.
At the high ground, we chose to sample the shrub areas with two different sampling methods, to see if this had any effect on the number and type of snails that we were able to find. The two sampling methods (See methods section) involved different placements of the line transects relative to one another. By statistically comparing the populations sampled at these, it was determined that there was a significant difference in the allele frequencies (χ2 = 10.06, P = 0.01). It is also significant when Yate’s correction is applied, because there is only one degree of freedom (χ2 = 8.27, P = 0.01). This could be due to the fact that the sampling methods were different, and the first covered less area than the second. In this way, human error and the skewed method may have contributed to the apparent difference in allele distribution. Our group was also the first to sample at the first site, eliminating interference from other groups, but it was possible that other groups had previously sampled at the second site.
However, the difference may be explained by the occurrence of genetic drift or selection. The sites were sufficiently far apart (distance between sites > average snail movement) that it can be concluded that the populations are isolated from one another. There were identifiable differences in the composition of the vegetation at each site. At the first site there were generally shorter plants with more soil disturbance, exposing the chalk, at the second site the plants were taller and there was a greater coverage of vegetation. These differences in conditions give ways in which selection might act upon the populations. For example the greater coverage of plants at the second site would give greater protection from predators and the accessibility of chalk at the first site would better enable shell development. It is almost certain that genetic drift and selection is action on both populations but without further investigation we cannot determine whether drift or selection has a greater effect on allele distribution and frequency.